Four main areas of work are proposed: 1. Spectrin sulfhydryl group activity and distribution. The number of spectrin sulfhydryls, and their location by subunit, will be determined by direct titration and with 14C n-ethyl maleimide, respectively. Unusually reactive groups will be looked for with 14C-NEM, and their location partially determined by controlled proteolytic fragmentation. Alteration of sulfhydryl activity by ATP, actin spectrin kinase and alkaline phosphates will be investigated. 2. If highly active, specifically titratable sulfhydryls are found, they will be spin-labelled, and the conformation of spectrin will be monitored as a function of salt concentration, pH, and the above agents. 3. Studies of actin polymerization state as a function of pH and of the presence of actin binding proteins (other than spectrin) will be performed, correlating spin label, electron microscope and ultracentrifuge data. 4. In parallel with the above studies, spectrin-actin interactions will be studies by electron microscopy and by ultracentrifugation, and correlated with previous EPR data.